A variety of pyroelectric infrared radiation detectors which can detect an object or temperature without direct contact have been utilized for measuring the temperature of an electronic oven, controlling an air conditioner to the room temperature, activating the opening and closing of an automatic door, triggering an alarm device, and so forth. Their range of applications will surely be increased in the coming future.
The pyroelectric infrared radiation detector is a sort of sensor using the pyroelectric effect of a ferroelectric substance. The ferroelectric substance has spontaneous polarization in one direction to generate positive and negative charges at the surface. In a common state under the atmospheric pressure, it remains neutral while coupling with charges of molecules in the atmosphere. It is known that every object emits an intensity of infrared ray according to the temperature. An infrared radiation sensor upon receiving at its probe an intensity of infrared radiation from an object causes its ferroelectric substance to produce an electricity corresponding to the thermal change by the radiation. For measuring the intensity of infrared radiation at higher accuracy, it is thus needed to have an infrared radiation detecting structure of the detector increased in the thermal response. This is implemented by using a thin film of a pyroelectric material.
One of the conventional pyroelectric infrared radiation detectors will be explained.
FIGS. 9(a), 9(b), and 9(c) are a plan view, a cross sectional view, and a processing flow chart respectively of the conventional pyroelectric infrared radiation detector. As shown in FIGS. 9(a) and 9(b), there are provided a single crystal substrate of magnesium oxide 91 (referred to as (100) MgO single crystal substrate hereinafter) and two electrodes 92a and 92b. The electrode 92b serves as an infrared radiation absorbing layer. Also, a pyroelectric thin film 93 is disposed between the two electrodes 92a and 92b in layers thus constituting an infrared radiation detecting structure. Denoted by 94a and 94b are polyimide resin layers for protecting and supporting the infrared radiation detecting structure or more particularly, main parts of the electrodes 92a and 92b. The resin layer 94a acts as an interlayer insulating layer between the two electrodes 92a and 92b. There is an opening 95 provided for reducing the thermal capacity of the pyroelectric thin film 93.
A procedure of producing the aforementioned conventional pyroelectric infrared radiation detector will now be explained referring to FIG. 9(c). The procedure starts with developing a layer of the pyroelectric thin film 93 on the (100) MgO single crystal substrate 91 by high-frequency magnetron sputtering of a material of titanate including lanthanum (referred to as PLT hereinafter) with metal masking.
Then, an interlayer insulating layer of the polyimid resin 94a is formed to about 1 .mu.m on the thin film and substrate. Using the magnetron sputtering technique, a 20-nm-thickness of nickel-chrome alloy (referred to as NiCr hereinafter) is coated as the electrode 92b on the (100) MgO single crystal substrate 91, the interlayer insulating layer 94a, and the pyroelectric thin film 93. The electrode 92b is finished when having been shaped to a predetermined pattern by photolithographic process. A layer of the polyimid 94b with about 3 .mu.m thickness is placed over the previous layers.
Etching process using phosphoric acid is applied through a masking to the lower surface of the (100) MgO single crystal substrate 91 in order to provide the opening 95 and expose the pyroelectric thin film 93. Then, a 200-nm-thick layer of the electrode 92a is formed on the etched surface of the (100) MgO signal crystal substrate 91 by sputtering of NiCr so that it connects to the pyroelectric thin film 93.
As understood, for the purpose of increasing the thermal sensitivity of the pyroelectric infrared radiation detector by improving the thermal response of the infrared radiation detecting structure, the contact area between the infrared radiation detecting structure and the (100) MgO single crystal substrate 91 has to be reduced considerably to minimize the thermal capacity of the infrared radiation detecting structure. This is achieved by providing the opening 95 partially in the (100) MgO single crystal substrate 91. However, if the opening 95 is increased in size for reduction of the thermal capacity, the pyroelectric thin film 93 and the polyimid layers 94a and 94b in the infrared radiation detecting structure tend to cause their inner distortion stresses to generate physical disconnection or breakage, thus decreasing the operational reliability. Also, the etching for making the opening 95 extending from lowermost to uppermost of the (100) MgO single crystal substrate 91 takes a considerable length of time lowering the productivity. In addition, the shape of the opening 95 is very likely oversized causing the size of the infrared radiation detector to increase.
Particularly, due to linear and/or two-dimensional arrangement of a multiplicity of the infrared radiation detectors, the polyimid resin layers often produce thermal cross-talk thereacross degrading the thermal response.